Oxygen sensor

ABSTRACT

A comparator element for gaseous oxygen comprises a first and a second oxygen-inert metal electrochemical cell. The first cell is fed with an oxygen containing gas of known oxygen content and the second cell is fed with a gas of unknown oxygen content. The design of the electrochemical cells is such that their electrical resistance when current is passed therethrough is inversely proportional to the oxygen content of the feed gas over a useful range. By comparing the resistances of the two cells, an accurate determination of the oxygen content of the unknown gas may be made with self compensation for ambient temperature. The range of the instrument may be increased by restricting the flow of gas to the oxygen electrodes. This may be accomplished by flowing the gas through a diffusion membrane and an orifice. Oxygen evolved from the metal electrode of the standard cell may be used to feed the oxygen electrode of the same cell thus providing a self sustaining standard atmosphere. The comparator may be compensated for pressure by providing pressure equalizing means between the gas feed to the first cell and the gas feed to the second.

ited States Patent [191 alaspina et al.

[ Jan. 22, 1974 OXYGEN SENSOR [75] Inventors: Francis P. Malaspina,Yardley;

Wesley E. Aker, Malvem, both of Pa.; John Werth, Princeton, NJ.

[52] US. Cl. 204/195 P, 204/1 T, 204/195 R,

204/195 M [51] Int. Cl.. G011! 27/26, GOlh 27/28, GOln 27/46 [58] Fieldof Search. 204/195 R, l T, 195 M, 195 P Primary ExaminerG. L. Kaplan[57] ABSTRACT A comparator element for gaseous oxygen comprises a firstand a second oxygen-inert metal electrochemical cell. The first cell isfed with an oxygen containing gas of known oxygen content and the secondcell is fed with a gas of unknown oxygen content. The design of theelectrochemical cells is such that their electrical resistance whencurrent is passed therethrough is inversely proportional to the oxygencontent of the feed v gas over a useful range. By comparing theresistances of the two cells, an accurate determination of the oxygencontent of the unknown gas may be made with self compensation forambient temperature. The range of the instrument may be increased byrestricting the flow of gas to the oxygen electrodes. This may beaccomplished by flowing the gas through a diffusion membrane and anorifice. Oxygen evolved from the metal electrode of the standard cellmay be used to feed the oxygen electrode of the same cell thus providinga self sustaining standard atmosphere. The comparator may be compensatedfor pressure by providing pressure equalizing means between the gas feedto the first cell and the gas feed to the second.

20 Claims, 6 Drawing Figures OXYGEN SENSOR BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to gasmonitoring and analysis devices. In particular it relates to devices formonitoring or analyzing the oxygen in a gas.

2. Description of the Prior Art The science of determining the oxygenconcentration in a gas has undergone a considerable evolution. One ofthe first oxygen deficiency sensors was the mine canary. A morescientific classical method is by chemical absorption such as isperformed in an Orsat gas analyzer. This is slow and not easily adaptedto continuous reading. Devices for measuring the thermal conductivity ofgas have been perfected. These provide continuous readings but require aconsiderable volume of gas for proper functioning.

Oxygen deficiency indicating devices are required when men must work inconfined spaces such as mines, vats, tanks, etc. Oxygen determinationapparatus is also a requisite in such diverse application as space shipsand capsules, submarines for military and civilian use as well ascombustion control and other environmental studies. In spite of the manydevices available, there is still a need for a small reliable oxygensensor and analyzer operable by unskilled people and capable of readinga wide range of oxygen concentration. The gas chromatograph is usefulfor analyzing the composition of even very minute gas samples. However,the chromatograph is a large and delicate instrument suitable forlaboratory work but not adapted to everyday use in industry. Smallchemical absorption tubes have been perfected to measure oxygen, butthese are of limited usefulness. Most recently fuel cell art has beenutilized to provide a device which measures the oxygen content of a gasby the output of an oxygen-fuel electrochemical cell. Such cells can bemade small in size, so that they may be carried about by a workerwithout inconvenience. However, they are subjected to loss ofcalibration due to the inconsistencies of the fuel cell.

SUMMARY OF THE INVENTION A comparator element for gaseous oxygencomprises a first and a second oxygen-inert metal electrochemical cell.The first cell is fed gas containing oxygen of known concentration andthe second cell is fed gas containing an unknown concentration ofoxygen. When current is passed through the cells, the polarizatio'nresistance provides a measure of the comparative concentration of oxygenin the feeds. The range of the device is extended by constricting theflow of gas to the cells. Oxygen evolved at the anode of the companioncell may be used to feed the cathode thereof thus providing a selfsustained reference. Compensation for ambient pressure may be providedby introducing pressure equalizing means between the two cells. Thecells may be made quite small, of the order of 1 sq. cm. electrode areaand requiring a feed in the order of 1/10 cc gas per second. A simpleelectriclal bridge circuit fed from a single dry cell is sufficient formost applications. This can be arranged to provide low or high leveloxygen alarms or may be used to directly read the percentage of oxygenpresent.

It will be seen from this description that the device of the inventioncan be made small in size, low in cost,

rugged and portable. It is ideally suited to wear by workers such asminers, divers astronauts, etc. The self compensation provided by theuse of two cells removes all problems usually associated withtemperature, age, etc., while the use of a gassing cathode rather than afuel cathode as used in certain presently available devices removes theuncertainty associated with the fuel electrode such as loss of catalyticpower, poisoning, as well as the need to carry a fuel supply.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts in cross section asimplified sensorreference cell of the invention;

FIG. 2 depicts the reference cell of FIG. 1 connected in a bridgecircuit for an oxygen sensor or alarm;

FIG. 3 depicts a performance curve of the sensor relating current flowthrough the cell with the content of oxygen fed to it;

FIG. 4 depicts a second embodiment of the sensorreference cell of theinvention;

FIG. 5 depicts a second form of electrical circuit for use with thesensor-reference cell of the invention; and FIG. 6 depicts the sensor ofthe invention connected for measuring gases from a chimney stack.

DESCRIPTION OF THE PREFERRED O ENTS,

In FIG. 1, 10 represents in cross section a cylindrical housingenclosing two electrochemical cell means. The housing might be made frominsulated metal or from one of the common structural plastics such asmethacrylate, styrene, etc. It must be non-reactive with the stronglyalkaline electrolyte. If a cemented construction is used, it should be acementable material, and it should be dimensionally stable. There arefour electrodes located in the housing, two to each cell means. Thefirst two of these, 12, are gassing electrodes made from an impervioussheet of inert metal such as nickel. They are held tightly in housing 10by ring 14 and are also sealed in position by a cement compatible withthe material of the housing10. The two electrodes separate the two cellcompartments l6 and 18. An electrical lead 20 is attached to eachelectrode 12. Two identical oxygen electrodes 22 and 24 form the endwalls of the compartments 16 and 18. Electrical leads 26 and 28 areattached to electrode 22 and 24. The electrodes can be any of the knownoxygen consuming electrode structures. An oxygen consuming electrodethat has been found particularly suitable can be made as follows: carbonpowder and Teflon (polytetrafluoroethylene) in the ratio of about fourto one by weight are mixed and formed into a porous sheet on rolls. Thesheet so formed is pressed onto an expanded nickel sheet or other gridstructure. A sheet of microporous non-wetting material such as Teflon(polytetrafluoroethylene) is then adhered by head and pressure to oneside of the above structure to form a triple laminate. In this electrodeof the microporous sheet side faces the gas to provide microporosity tothe electrode and to provide an hydrophobic surface to prevent leakageof electrolyte therethrough. The carbon particles provide catalyticsites and the screen serves as a current collector.

The two cell compartments 16 and 18 are filled with a suitable alkalineelectrolyte such as potassium hydroxide solution. Covered vents 30 and32 provide access to the two compartments.

FIG. 2 shows a typical bridge circuit using the sensor of FIG. 1. 16 and18 represent the two cell compartments and the two cell means. Resistors44 and 46 connect the two air electrodes 22 and 24 to the negative ofbattery 48. Battery 48 need be only a single cell as 1.3 volts has beenfound to be ample for driving the circuit. For convenience, one or bothresistors 44 and 46 may be adjustable. The adjustable feature is usefulin calibrating the instrument. To complete the bridge circuit asignalling device such as the meter 50 is connected to the electricalleads of the gas electrodes. Other signaling devices include means suchas a light or bell. Alternately an activator for a value or damper maybe caused to operate from the bridge circuit.

It is to be noted that when a direct current power source of fixedvoltage is connected to cell means such as shown in FIG. 2 with thepositive of the source connected to the nickel electrode and thenegative of the source connected to the gas electrode, the current flowthrough the cell will be directly proportional to the oxygen availableat the gas electrode. FIG. 3 shows a typical current oxygen curve forsuch a cell. Above about percent oxygen the cell becomes saturated withoxygen and the proportionality relationship no longer holds. The cellcan be likened to a variable resistor whose resistance value within theoperating range is inversely proportioned to the oxygen content.

To use the sensor in the bridge circuit of FIG. 2, one of the cells,say, 22, is exposed to an oxygen source of a known concentration and theother cell, 24 is exposed to a source of unknown or varying oxygencontent. When the concentration of oxygen of the unknown is equal tothat of the known source, the bridge will be balanced; otherwise it willbe out of balance. The device described will only be useful formeasuring or comparing oxygen concentration below about 5 percent. Abovethis, the cell becomes insensitive as shown in curve FIG. 3. As a meansto overcome this limitation, a constriction can be put in the linefeeding the known and unknown gasses to the cell. Although various formsof orifice could be used, it has been found preferably to use acombination of an orifice and a diffusion membrane. The diffusionmembrane should be a true diffusion membrane rather than a porousscreen. Thin sections (5 to mils thick) of such materials as natural andsilicone rubber, polytetrafluoroethylene, fluorinated ethylene-propyleneco-polymer, polybutadiene, poly(butadiene-styrene) ethyl cellulose,cellulose acetate and protein enriched triacetate cellulose makeexcellent membranes. Excellent results have been found when the membraneis chosen from such materials as silicone rubber, or protein enrichedtriacetate cellulose in thin sections. By the use of the orifice anddiffusion membrane, the range of the oxygen sensing cells can beextended to cover the entire range of zero to 100 percent oxygen. FIG. 4shows a cell design having diffusion membranes 60 and orifices 62located in the passage feeding the gas electrodes 22 and 24. This cellalso uses a single anode 13 for simplicity.

A further refinement of the reference cell is shown in dotted lines inFIG. 4. It has been noted that oxygen is produced from the electrode 13.The amount of oxygen given off in each cell must be axactly equal to theoxygen consumed by the oxygen electrodes. A conduit 64 is shown carryingoxygen from the cell compartment 16 to a chamber 66 feeding orifice 62,difiusion membrane and gas electrode 22. With this construction theoxygen concentration in the cell defined by compartment l6 and electrode22 will remain constant and at the value which was present when the cellis put in operation. Thus the cell means with the oxygen return conduitbecomes an excellent reference cell. It is self contained and can beoperated in any environment without affecting its output.

Another feature of the sensor is that by providing a means forequalizing the pressure between compartments 66 and 64, the sensorbecomes self compensating for pressure. When the sensor is to be used inthe open, i.e., with the complete device immersed in the gas of unknownoxygen content, a simple pressure equalizing means is to have a flexiblegas bag attached to the feed-back tube 64. Where cell 24 is fed from aclosed system, an enclosed bellows device may be used. Because bothcells are closely associated and can be made conveniently small, thesensor has an automatic compensation for temperature.

The electrical circuit of FIG. 2 is very convenient when the sensor isused for an oxygen level control or alarm. For such use, the bridgebalance point is adjusted by means of a variable resistance 46 so thatthe null point of the bridge is at the oxygen level which it is desiredto monitor. As long as the oxygen level stays on the safe side of thenull point, the polarity of the electric potential at meter 50 will bein one direction. However, if the level crosses into the danger area,the potential will reverse. Means for converting the reverse signal intoa control or alarm signal are well known; for example a reverse voltagerelay or an operational amplifier could be placed at 50 to give a powersignal capable of operating a valve or energizing a light bulb or horn.

Because of th linearity of the cell potential with respect to oxygencontent of the sensed gas, the sensor cell may be used as a directreading oxygen meter. A possible circuit for this purpose is shown inFIG. 5. This circuit is similar to the circuit of FIG. 2 except that ameter 70 calibrated in percent oxygen having a paralleled variableresistance shunt 72 and a variable resistor 74 make up the resistor ofthe bridge indicated by 46 of FIG. 2. To operate the oxygen meter, thetwo sides of the sensor are fed from the same gas supply of known oxygencontent. A convenient reference gas is air of 21 percent oxygen. Thebridge is then balanced by combined adjustment of resistors 72 and 74.When balanced, meter 50 will be at its null point and meter 70 willpoint to the percent oxygen in the standard gas. To measure a gas ofunknown oxygen content, cell 18 is fed the unknown gas and cell 16 isfed with the standard gas. The percent oxygen will be proportional tothe current flow through the cell 18.

It will be observed that the oxygen sensor of this invention is capableof being built in a small size and in fact by making it small the lagtime required to reach equilibrium will be made small; the electricalcircuiting can likewise be made small. It is self contained and can beenergized by a single dry cell. It is also rugged and truly portable.Thus it differs considerably from the classical oxygen sensing andmeasuring devices such as the orsat, the thermal cell and the gaschromatograph. These features make it particularly adaptable as apersonal oxygen detector for miners, resuce workers and people workingin enclosed spaces. Because of its builtin pressure compensation, it issuitable for deep diving work where a small simple direct reading deviceis of extreme importance. For the same reason it is suitable foraerospace applications where pressures are below atmospheric. The deviceis useful in the process and other industries where control ormeasurement of oxy gen content is required. Unfortunately, theelectrolyte of the cell of the invention is a reactive to a gas such ascarbon dioxide. Thus where such gasses may be present as in a flue gasit is necessary to pass the gas through a carbon dioxide absorptioncolumn prior to feeding it to the sensor cell.

When an unknown gas is fed to the sensor via a tube, it is necessarythat the atmosphere to which the sensor is exposed is not materiallychanged by the oxygen consumed by the sensor. To achieve this, it isdesirable to have a flow of gas across the sensor area. As an example,FIG. 6 shows chimney 80 connected by tube 8 2 to carbon dioxideabsorption tube 84. Tube 86 feeds the washed gas to the sensor side 88of gas analyzer means 90. A continuous flow of flue gas through thesystem is provided by a gas pump as for instance the aspirator 92. Thestandard side 94 of the analyzer 90 receives a continuous supply ofoxygen from its gassing electrode via the feed back tube 96. A pressureequalizer 100 comprising a sealed vessel 102 with an internal flexiblewall 104 is provided to equalize the gas pressures between cell means 88and 94 by means of tubes 106 and 108 respectively.

Other uses of the device of the invention will be obvious to thoseskilled in the art of gas sensing and analy- SIS.

Having fully described our invention and given examples of itsembodiment as well as pointing out numerous sites where it will be ofuse, we claim:

1. A comparator for comparing the oxygen concentration of a gas ofunknown oxygen content with the oxygen concentration of a gas of knownoxygen content which comprises:

a. a first electrochemical resistance cell comprising a first oxygenconsuming cathode receiving a first gas of known oxygen concentration, afirst electrolyte and an inert metallic gassing anode;

b. a second electrochemical resistance cell comprising a second oxygenconsuming cathode receiving a second gas of unknown oxygenconcentration, a second electrolyte and an inert metallic gassing anode;

c. a housing containing the first electrochemical cell and the secondelectrochemical cell; means for supplying a positive current to thecathode of the first electrochemical cell to produce a first resistivedrop across the first electrochemical cell the first drop beingdependent upon the oxygen concentration of the first gas;

e. means for supplying a positive current to the cathode of the secondelectrochemical cell to produce a second resistive drop across thesecond electrochemical cell the second drop being dependent upon theoxygen concentration of the second gas;

2. A comparator as defined in claim ll including a first and a secondmeans for restricting gas How, the first means for restricting gas flowrestricting the flow of gas to the cathode of the first electrochemicalcell and the second means for restricting gas flow restricting the flowof gas to the cathode and the second electrochemical cell.

3. A comparator as defined in claim 2 wherein the first means forrestricting the flow of gas to the first cathode includes a firstdiffusion membrane and the second means for restricting the flow of gasto the second cathode includes a second diffusion membrane.

4. A comparator as defined in claim 3 wherein the material from whichthe first and the second diffusion membranes are made is selected fromthe group which consists of natural rubber, silicone rubber,polytetrafluoroethylene, fluorinated ethylene-propylene copolymer,polybutadiene, poly(butadiene-styrene), ethyl cellulose, celluloseacetate, and protein enriched triacetate cellulose.

5. A comparator as defined in claim 2 wherein the first means forrestricting the flow of gas to the first cathode includes a firstorifice and the second means for restricting the flow of gas to thesecond cathode includes a second orifice.

6. A comparator as defined in claim 1 including conduit means forconducting oxygen gas given off by the anode of the firstelectrochemical cell to the cathode thereof.

7. A comparator as defined in claim 6 including means equalizing thepressure within the first electrochemical resistance cell to thepressure within the second electrochemical resistance cell.

8. A comparator as defined in claim 1 wherein each oxygen gas consumingcathode comprises a laminate of metallic grid, a porous sheet of carbonand polytetrafluoroethylene, and a sheet of microporouspolytetrafluoroethylene, the side of the laminate bearing the grid beingexposed to the electrolyte of the cell and the side of the laminatebearing the microporous sheet being exposable to an oxygen containinggas.

9. A comparator as defined in claim 1 wherein the means for comparingthe resistance defined by the second drop with the resistance defined bythe first drop comprises a four armed electrical bridge circuit.

10. A comparator element as defined in claim 1 wherein the inertmetallic anode of the first electrochemical cell is a first side of ametallic anode and the inert metallic anode of the secondelectrochemical cell in the second side of said metallic anode.

11. An oxygen meter for determining the oxygen concentration of a gas ofunknown oxygen content which comprises:

a. first electrochemical resistance cell comprising a first oxygenconsuming cathode receiving a first gas of known oxygen concentration, afirst electrolyte and an inert metallic gassing anode;

b. a second electrochemical resistance cell comprising a second oxygenconsuming cathode receiving a second gas of unknown oxygenconcentration, a second electrolyte and an inert metallic gassing anode;

c. a housing containing the first electrochemical cell and the secondelectrochemical cell;

' d. means for supplying a positive current to the cathode of the firstelectrochemical cell to produce a first resistive drop across the firstelectrochemical cell the first drop being dependent upon the oxygenconcentration of the first gas;

e. means for supplying a positive current to the cathode of the secondelectrochemical cell to produce a second resistive drop across thesecond electrochemical cell the second drop being dependent upon theoxygen concentration of the second gas;

f. means for supplying an equal and predetermined positive potential tothe cathode of the first electrochemical cell and to the cathode of thesecond l reshwisa ss V.

g. means for measuring the flow of current to the 9sth9 9 the $99M sqqshsm 521 9 7 h. calibration means for directly relating the magnitudeof the flow of current to the oxygen concentration of the gas of unknownoxygen content.

12. An oxygen meter as defined in claim 1 1 including a first and asecond means for restricting gas flow, the first means for restrictinggas flow restricting the flow of gas to the cathode of the firstelectrochemical cell and the second means for restricting gas flowrestricting the flow of gas to the cathode of the second electrochemicalcell.

13. An oxygen meter as defined in claim 12 wherein the first means forrestricting the flow of gas to the first cathode includes a firstdiffusion membrane and the second means for restricting the flow of gasto the second cathode includes a second diffusion membrane.

14. An oxygen meter as defined in claim 13 wherein the material fromwhich the first and the second diffusion membranes are made is selectedfrom the group which consists of natural rubber, silicone rubber,polytetrafluoroethylene, fluorinated ethylene-propylene co-polymer,polybutadiene, poly(butadiene-styrene),

ethyl cellulose, cellulose acetate, and protein enriched triacetatecellulose.

15. An oxygen meter as defined in claim 12 wherein the first means forrestricting the flow of gas to the first cathode includes a firstorifice and the second means for restricting the flow of gas to thesecond cathode includes a second orifice.

16. An oxygen meter as defined in claim 1 1 including conduit means forconducting oxygen gas given off by the anode of the firstelectrochemical cell to the cathode thereof.

17. An oxygen meter as defined in claim 16 including means equalizingthe pressure within the first electrochemical resistance cell to thepressure within the second electrochemical resistance cell.

18. An oxygen meter as defined in claim 11 wherein each oxygen gasconsuming cathode comprises a laminate of a metallic grid, a poroussheet of carbon and polytetrafluoroethylene, and a sheet of microporouspolytetrafluoroethylene, the side of the laminate bearing the grid beingexposed to the electrolyte of the cell and the side of the laminatebearing the microporous sheet being exposable to an oxygen containinggas.

19. An oxygen meter as defined in claim 11 wherein the means forsupplying a positive potential includes an electrochemical battery, andthe means for measuring the current flow including a four armedelectrical bridge circuit.

20. An oxygen meter as defined in claim 11 wherein the inert metallicanode of the first electrochemical cell is a first side of a metallicanode and the inert metallic anode of the second electrochemical cell isthe second side of said metallic anode.

2. A comparator as defined in claim 1 including a first and a secondmeans for restricting gas flow, the first means for restricting gas flowrestricting the flow of gas to the cathode of the first electrochemicalcell and the second means for restricting gas flow restricting the flowof gas to the cathode and the second electrochemical cell.
 3. Acomparator as defined in claim 2 wherein the first means for restrictingthe flow of gas to the first cathode includes a first diffusion membraneand the second means for restricting the flow of gas to the secondcathode includes a second diffusion membrane.
 4. A comparator as definedin claim 3 wherein the material from which the first and the seconddiffusion membranes are made is selected from the group which consistsof natural rubber, silicone rubber, polytetrafluoroethylene, fluorinatedethylene-propylene co-polymer, polybutadiene, poly(butadiene-styrene),ethyl cellulose, cellulose acetate, and protein enriched triacetatecellulose.
 5. A comparator as defined in claim 2 wherein the first meansfor restricting the flow of gas to the first cathode includes a firstorifice and the second means for restricting the flow of gas to thesecond cathode includes a second orifice.
 6. A comparator as defined inclaim 1 including conduit means foR conducting oxygen gas given off bythe anode of the first electrochemical cell to the cathode thereof.
 7. Acomparator as defined in claim 6 including means equalizing the pressurewithin the first electrochemical resistance cell to the pressure withinthe second electrochemical resistance cell.
 8. A comparator as definedin claim 1 wherein each oxygen gas consuming cathode comprises alaminate of metallic grid, a porous sheet of carbon andpolytetrafluoroethylene, and a sheet of microporouspolytetrafluoroethylene, the side of the laminate bearing the grid beingexposed to the electrolyte of the cell and the side of the laminatebearing the microporous sheet being exposable to an oxygen containinggas.
 9. A comparator as defined in claim 1 wherein the means forcomparing the resistance defined by the second drop with the resistancedefined by the first drop comprises a four armed electrical bridgecircuit.
 10. A comparator element as defined in claim 1 wherein theinert metallic anode of the first electrochemical cell is a first sideof a metallic anode and the inert metallic anode of the secondelectrochemical cell in the second side of said metallic anode.
 11. Anoxygen meter for determining the oxygen concentration of a gas ofunknown oxygen content which comprises: a. first electrochemicalresistance cell comprising a first oxygen consuming cathode receiving afirst gas of known oxygen concentration, a first electrolyte and aninert metallic gassing anode; b. a second electrochemical resistancecell comprising a second oxygen consuming cathode receiving a second gasof unknown oxygen concentration, a second electrolyte and an inertmetallic gassing anode; c. a housing containing the firstelectrochemical cell and the second electrochemical cell; d. means forsupplying a positive current to the cathode of the first electrochemicalcell to produce a first resistive drop across the first electrochemicalcell the first drop being dependent upon the oxygen concentration of thefirst gas; e. means for supplying a positive current to the cathode ofthe second electrochemical cell to produce a second resistive dropacross the second electrochemical cell the second drop being dependentupon the oxygen concentration of the second gas; f. means for supplyingan equal and predetermined positive potential to the cathode of thefirst electrochemical cell and to the cathode of the secondelectrochemical cell; g. means for measuring the flow of current to thecathode of the second electrochemical cell; and h. calibration means fordirectly relating the magnitude of the flow of current to the oxygenconcentration of the gas of unknown oxygen content.
 12. An oxygen meteras defined in claim 11 including a first and a second means forrestricting gas flow, the first means for restricting gas flowrestricting the flow of gas to the cathode of the first electrochemicalcell and the second means for restricting gas flow restricting the flowof gas to the cathode of the second electrochemical cell.
 13. An oxygenmeter as defined in claim 12 wherein the first means for restricting theflow of gas to the first cathode includes a first diffusion membrane andthe second means for restricting the flow of gas to the second cathodeincludes a second diffusion membrane.
 14. An oxygen meter as defined inclaim 13 wherein the material from which the first and the seconddiffusion membranes are made is selected from the group which consistsof natural rubber, silicone rubber, polytetrafluoroethylene, fluorinatedethylene-propylene co-polymer, polybutadiene, poly(butadiene-styrene),ethyl cellulose, cellulose acetate, and protein enriched triacetatecellulose.
 15. An oxygen meter as defined in claim 12 wherein the firstmeans for restricting the flow of gas to the first cathode includes afirst orifice and the second means for restricting the flow of gas tothe second cathode includes a seCond orifice.
 16. An oxygen meter asdefined in claim 11 including conduit means for conducting oxygen gasgiven off by the anode of the first electrochemical cell to the cathodethereof.
 17. An oxygen meter as defined in claim 16 including meansequalizing the pressure within the first electrochemical resistance cellto the pressure within the second electrochemical resistance cell. 18.An oxygen meter as defined in claim 11 wherein each oxygen gas consumingcathode comprises a laminate of a metallic grid, a porous sheet ofcarbon and polytetrafluoroethylene, and a sheet of microporouspolytetrafluoroethylene, the side of the laminate bearing the grid beingexposed to the electrolyte of the cell and the side of the laminatebearing the microporous sheet being exposable to an oxygen containinggas.
 19. An oxygen meter as defined in claim 11 wherein the means forsupplying a positive potential includes an electrochemical battery, andthe means for measuring the current flow including a four armedelectrical bridge circuit.
 20. An oxygen meter as defined in claim 11wherein the inert metallic anode of the first electrochemical cell is afirst side of a metallic anode and the inert metallic anode of thesecond electrochemical cell is the second side of said metallic anode.